U.S. patent number 7,524,473 [Application Number 11/726,804] was granted by the patent office on 2009-04-28 for method of mercury removal in a wet flue gas desulfurization system.
This patent grant is currently assigned to Alstom Technology Ltd. Invention is credited to Fredrik J. Brogaard, Leif A. V. Lindau.
United States Patent |
7,524,473 |
Lindau , et al. |
April 28, 2009 |
Method of mercury removal in a wet flue gas desulfurization
system
Abstract
Controlling the reductive capacity of an aqueous alkaline slurry
(23) in a wet scrubber makes it possible to accurately control the
mercury emission from the scrubber to a desired value. One method
of controlling the reductive capacity of the slurry is to measure
the reduction-oxidation potential ("redox potential") of the
aqueous alkaline slurry (23) and to add or remove substances that
affect the redox potential and thus the reductive capacity of the
slurry. In wet scrubbers in which limestone is used for absorption
of acid gases and where a gypsum slurry is circulated, it has been
found to be an attractive solution to control the amount of
oxidation air blown into the scrubber in order to control the redox
potential and thereby the mercury emissions.
Inventors: |
Lindau; Leif A. V. (Arlov,
SE), Brogaard; Fredrik J. (Vaxjo, SE) |
Assignee: |
Alstom Technology Ltd (Baden,
CH)
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Family
ID: |
39598431 |
Appl.
No.: |
11/726,804 |
Filed: |
March 23, 2007 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20080233024 A1 |
Sep 25, 2008 |
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Current U.S.
Class: |
423/210;
423/243.08; 423/244.08; 423/DIG.5; 423/244.07; 422/168; 422/108;
422/110; 422/111; 422/105 |
Current CPC
Class: |
B01D
53/346 (20130101); B01D 53/501 (20130101); F23J
15/04 (20130101); B01D 53/64 (20130101); Y10S
423/05 (20130101); B01D 2251/102 (20130101); B01D
2257/602 (20130101); F23J 2215/20 (20130101); F23J
2215/60 (20130101); F23J 2219/40 (20130101) |
Current International
Class: |
B01D
53/50 (20060101); B01D 53/64 (20060101); B01D
53/74 (20060101); G05B 1/00 (20060101); G05D
21/00 (20060101) |
Field of
Search: |
;423/210,243.08,244.07,244.08,DIG.5 ;422/105,108,110,111,168 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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95/33547 |
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Dec 1995 |
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WO |
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96/14137 |
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May 1996 |
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WO |
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Primary Examiner: Vanoy; Timothy C
Attorney, Agent or Firm: Wiggin and Dana LLP Gangemi;
Anthony P.
Claims
What is claimed is:
1. A method for controlling an amount of mercury discharged to an
environment in a flue gas generated by combustion of a fuel source,
said method comprising: subjecting said flue gas to a wet scrubbing
operation to decrease an amount of sulfur oxides present in said
flue gas, said wet scrubbing operation comprising contacting said
flue gas with an aqueous alkaline slurry to absorb said sulfur
oxides from said flue gas, wherein at least a portion of gaseous
ionic mercury species present in said flue gas are dissolved in
said aqueous alkaline slurry and thereby removed from the flue gas;
measuring a redox potential of the aqueous alkaline slurry used in
the wet scrubbing operation to provide a signal indicative of the
measured redox potential; and controlling the amount of ionic
mercury present in flue gas that can be reduced to elemental
mercury by the slurry by adjusting the redox potential of the
aqueous alkaline slurry used in the wet scrubbing operation in
response to the signal.
2. The method of claim 1, wherein said aqueous alkaline slurry
comprises at least one of: lime, limestone and calcium sulfate.
3. The method of claim 1, wherein adjusting the redox potential of
the aqueous alkaline slurry comprises: comparing the measured redox
potential of said aqueous alkaline slurry to a predetermined redox
potential.
4. The method of claim 3, further comprising: measuring an amount
of gaseous elemental mercury emitted from a scrubber tower to
provide a signal indicative of the measured amount of gaseous
elemental mercury; and determining the predetermined redox
potential in response to the signal indicative of the measured
amount of gaseous elemental mercury.
5. The method of claim 3, wherein the predetermined redox potential
is between about 100 mv and about 600 mv.
6. The method of claim 1, wherein the predetermined redox potential
is part of a range of predetermined redox potential values.
7. The method of claim 1, wherein adjusting the redox potential of
the aqueous alkaline slurry comprises: adjusting an amount of
oxygen containing gas introduced to said aqueous alkaline slurry,
wherein said oxygen containing gas affects the redox potential of
said aqueous alkaline slurry.
8. The method of claim 1, wherein adjusting the redox potential of
the aqueous alkaline slurry comprises: measuring an amount of
gaseous elemental mercury emitted from a scrubber tower to provide
a signal indicative of the measured amount of gaseous elemental
mercury; determining a redox potential value in response to the
signal indicative of the measured amount of gaseous elemental
mercury to provide a predetermined redox potential; comparing the
measured redox potential of said aqueous alkaline slurry to the
predetermined redox potential; and adjusting an amount of oxygen
containing gas introduced to said aqueous alkaline slurry in
response to the comparison of the measured redox potential of said
aqueous alkaline slurry to the predetermined redox potential,
wherein said oxygen containing gas affects the redox potential of
said aqueous alkaline slurry.
9. A method for controlling an amount of mercury discharged to an
environment in a flue gas generated by combustion of a fuel source,
said method comprising: subjecting said flue gas to a wet scrubbing
operation to decrease an amount of sulfur oxides present in said
flue gas, said wet scrubbing operation comprising contacting said
flue gas with an aqueous alkaline slurry to absorb said sulfur
oxides from said flue gas, wherein at least a portion of gaseous
ionic mercury species present in said flue gas are dissolved in
said aqueous alkaline slurry and thereby removed from the flue gas;
measuring an amount of gaseous elemental mercury emitted from a
scrubber tower to provide a signal indicative of the measured
amount of gaseous elemental mercury; and adjusting a redox
potential of the aqueous alkaline slurry used in the wet scrubbing
operation using the signal, thereby controlling the amount of ionic
mercury present in flue gas that can be reduced to elemental
mercury by the slurry.
10. The method of claim 9, wherein said aqueous alkaline slurry
comprises at least one of: lime, limestone and calcium sulfate.
11. The method of claim 9, wherein adjusting the redox potential of
the aqueous alkaline slurry comprises: determining a redox
potential value in response to the signal indicative of the
measured amount of gaseous elemental mercury to provide a
predetermined redox potential; measuring a redox potential of the
aqueous alkaline slurry used in the wet scrubbing operation;
comparing the measured redox potential of said aqueous alkaline
slurry to the predetermined redox potential; and adjusting an
amount of oxygen containing gas introduced to said aqueous alkaline
slurry in response to the comparison of the measured redox
potential of said aqueous alkaline slurry to the predetermined
redox potential, wherein said oxygen containing gas affects the
redox potential of said aqueous alkaline slurry.
12. The method of claim 11, wherein the predetermined redox
potential is part of a range of predetermined redox potential
values.
13. A system for controlling an amount of mercury discharged to an
environment in a flue gas generated by combustion of a fuel source,
said system comprising: a scrubbing tower in which the flue gas is
subjected to an aqueous alkaline slurry to decrease an amount of
sulfur oxides present in said flue gas, wherein at least a portion
of gaseous ionic mercury species present in said flue gas are
dissolved in the aqueous alkaline slurry and thereby removed from
the flue gas, the scrubbing tower including a collecting tank to
collect the aqueous alkaline slurry used in the wet scrubbing
operation; a measuring device coupled to the collecting tank and
configured to provide a signal indicative of a redox potential of
the aqueous alkaline slurry used in the wet scrubbing operation;
and a means for controlling the amount of ionic mercury present in
the flue gas that can be reduced to elemental mercury by the
slurry, whereby the redox potential of the aqueous alkaline slurry
used in the wet scrubbing operation is adjusted in response to the
signal.
14. The system of claim 13, wherein said aqueous alkaline slurry
comprises at least one of: lime, limestone and calcium sulfate.
15. The system of claim 13, wherein the means for controlling the
amount of ionic mercury present in the flue gas that can be reduced
to elemental mercury by the slurry includes: a forced oxidation
system coupled to the collecting tank; and a controller configured
to adjust an amount of oxygen containing gas introduced to the
collecting tank by the forced oxidation system in response to the
signal.
16. The system of claim 15, wherein the controller is further
configured to compare of the measured redox potential of said
aqueous alkaline slurry to a predetermined redox potential and
provide a control signal to the forced oxidation system in response
to the comparison.
17. The system of claim 16, further comprising: a mercury
measurement device coupled to a flue gas outlet of the scrubbing
tower and configured to provide a signal indicative of an amount of
gaseous elemental mercury emitted from the scrubber tower; and
wherein the controller is further configured to determine the
predetermined redox potential in response to the signal indicative
of an amount of gaseous elemental mercury emitted from the scrubber
tower.
18. The system of claim 16, wherein the predetermined redox
potential is between about 100 mv and about 600 mv.
19. The system of claim 16, wherein the controller is further
configured to determine the predetermined redox potential using at
least one of boiler load and coal quality.
Description
BACKGROUND
(1) Field
The disclosed subject matter generally relates to controlling an
amount of mercury discharged to an environment incident to the
combustion of a fuel source containing mercury or mercury
compounds, and more particularly to controlling the mercury
discharge in a combustion flue gas which is subjected to a wet
scrubbing operation.
(2) Description of the Related Art
Combustion of fuel sources such as coal produces a waste gas,
referred to as "flue gas" that is to be emitted into an
environment, such as the atmosphere. The fuel sources typically
contain sulfur and sulfur compounds which are converted in the
combustion process to gaseous species, including sulfur oxides,
which then exist as such in the resulting flue gas. The fuel
sources typically also contain elemental mercury or mercury
compounds which are converted in the combustion process to, and
exist in the flue gas as, gaseous elemental mercury or gaseous
ionic mercury species.
Accordingly, flue gas contains particles, noxious substances and
other impurities that are considered to be environmental
contaminants. Prior to being emitted into the atmosphere via a
smoke stack ("stack"), the flue gas undergoes a cleansing or
purification process. In coal combustion, one aspect of this
purification process is normally a desulfurization system, such as
a wet scrubbing operation known as a wet flue gas desulfurization
(WFGD) system.
Sulfur oxides are removed from flue gas using a WFGD system by
introducing an aqueous alkaline slurry to a scrubber tower of the
WFGD system. The aqueous alkaline slurry typically includes a basic
material that will interact with contaminants to remove them from
the flue gas. Examples of basic materials that are useful in the
aqueous alkaline slurry include, but are not limited to: lime,
limestone, magnesium, calcium sulfate, and the like, and
combinations thereof.
Recently, there has been an increased focus on the removal of
mercury. Presently, there are various methods for removing mercury
from flue gas. Those methods include, but are not limited to:
addition of oxidizing agents in a boiler upstream of the flue gas
emission control system and then removing it with scrubbers;
addition of reactants to bind mercury and remove it from the flue
gas; and utilization of particular coal or fuel that minimizes the
amount of mercury released when the coal or fuel is burned.
It has been shown that a number of generally known methods of
mercury removal are effective to produce mercury salts, which can
be dissolved and removed by the aqueous alkaline slurry used in the
wet scrubbing operation. Some of these methods include the addition
of halogen or halogen compounds, such as bromine, to the coal or to
the flue gas upstream of the wet scrubbing operation, to provide
oxidation of elemental mercury to ionic mercury and formation of
mercury salts, which are then dissolved in the aqueous alkaline
slurry incident to the sulfur oxide removal processes. However, the
removal of mercury in the aqueous alkaline slurry of a wet scrubber
has proven to be difficult to control and it is not easily
predicted when designing a flue gas cleaning system with respect to
mercury removal. The desired emission guarantee levels are often as
low as 1 mg/Nm.sup.3 of mercury, which corresponds to a very high
mercury removal efficiency in the wet scrubber.
BRIEF SUMMARY
One aspect of the disclosed subject matter relates to a method for
controlling an amount of mercury discharged to an environment in a
flue gas generated by combustion of a fuel source. The method
includes subjecting the flue gas to a wet scrubbing operation to
decrease an amount of sulfur oxides present in the flue gas, the
wet scrubbing operation comprising contacting the flue gas with an
aqueous alkaline slurry to absorb the sulfur oxides from the flue
gas, wherein at least a portion of gaseous ionic mercury species
present in the flue gas are dissolved in the aqueous alkaline
slurry and thereby removed from the flue gas, measuring a redox
potential of the aqueous alkaline slurry used in the wet scrubbing
operation to provide a signal indicative of the measured redox
potential and adjusting the redox potential of the aqueous alkaline
slurry used in the wet scrubbing operation in response to the
signal, thereby controlling the amount of ionic mercury present in
flue gas that can be reduced to elemental mercury by the
slurry.
Another aspect of the disclosed subject matter relates to a method
for controlling an amount of mercury discharged to an environment
in a flue gas generated by combustion of a fuel source. The method
includes subjecting the flue gas to a wet scrubbing operation to
decrease an amount of sulfur oxides present in the flue gas, the
wet scrubbing operation includes contacting the flue gas with an
aqueous alkaline slurry to absorb the sulfur oxides from the flue
gas, wherein at least a portion of gaseous ionic mercury species
present in the flue gas are dissolved in the aqueous alkaline
slurry and thereby removed from the flue gas. Measuring an amount
of gaseous elemental mercury emitted from a scrubber tower to
provide a signal indicative of the measured amount of gaseous
elemental mercury and adjusting a redox potential of the aqueous
alkaline slurry used in the wet scrubbing operation using the
signal, thereby controlling the amount of ionic mercury present in
flue gas that can be reduced to elemental mercury by the
slurry.
Another aspect of the disclosed subject matter relates to a system
for controlling an amount of mercury discharged to an environment
in a flue gas generated by combustion of a fuel source. The system
includes a scrubbing tower in which the flue gas is subjected to an
aqueous alkaline slurry to decrease an amount of sulfur oxides
present in the flue gas, wherein at least a portion of gaseous
ionic mercury species present in the flue gas are dissolved in the
aqueous alkaline slurry and thereby removed from the flue gas, the
scrubbing tower includes a collecting tank to collect the aqueous
alkaline slurry used in the wet scrubbing operation. A measuring
device coupled to the collecting tank and configured to provide a
signal indicative of a redox potential of the aqueous alkaline
slurry used in the wet scrubbing operation and means for adjusting
the redox potential of the aqueous alkaline slurry used in the wet
scrubbing operation in response to the signal, thereby controlling
the amount of ionic mercury present in flue gas that can be reduced
to elemental mercury by the slurry.
The details of one or more embodiments are set forth in the
accompanying drawing and the description below. Other features,
objects and advantages will be apparent from the description and
drawing, and from the claims.
BRIEF DESCRIPTION OF THE DRAWINGS
For the purpose of illustrating the subject matter disclosed
herein, the drawing shows a form of the embodiments that is
presently preferred. However, it should be understood that the
disclosed subject matter is not limited to the precise arrangements
and instrumentalities shown in the drawing, wherein:
FIG. 1 is a schematic representation of a system for controlling an
amount of gaseous elemental mercury emitted by a flue gas, which is
practiced using a wet scrubber.
DETAILED DESCRIPTION
The present inventors have discovered that controlling the
reductive capacity of an aqueous alkaline slurry in a wet scrubber
makes it possible to accurately control the mercury emission from
the scrubber to a desired value. As used herein, the "reductive
capacity" is the amount of ionic mercury present in flue gas that
can be reduced to elemental mercury by the slurry. One method of
controlling the reductive capacity of the slurry is to measure the
reduction-oxidation potential ("redox potential") of the aqueous
alkaline slurry and to add or remove substances that affect the
redox potential and thus the reductive capacity of the slurry. In
wet scrubbers in which limestone is used for absorption of acid
gases and where a gypsum slurry is circulated, it has been found to
be an attractive solution to control the amount of oxidation air
blown into the scrubber in order to control the redox potential and
thereby control the mercury emissions. If it is desired to increase
the emission of mercury the amount of oxidation air is controlled
to a lower amount, which results in a lower redox potential and a
higher emission of mercury. If, on the other hand, the mercury
emission becomes too high, the amount of oxidation air is
controlled to a higher amount, which results in a higher redox
potential and a lower emission of mercury. In this manner, it is
possible to stay below a maximum allowed emission of mercury with a
minimum consumption of oxidation air. Further the emission of
mercury becomes controllable and predictable such that a guarantee
for mercury emission value can be based on the capability of
removing mercury in the scrubber.
Referring now to FIG. 1, one example of a system for controlling an
amount of gaseous elemental mercury emitted by a flue gas, which is
practiced using a wet scrubbing operation, is shown generally at
10. In system 10, a flue gas 20 travels from a combustion source,
such as a coal-fired boiler, and enters a scrubber tower 22 through
inlet 24. While scrubber tower 22 is shown in one form, it is
contemplated that other forms of scrubber towers can be used in
conjunction with the present invention.
Once inside scrubber tower 22, flue gas 20 comes into contact with,
among other things, an aqueous alkaline slurry 23 to remove
contaminants from the flue gas 20. Aqueous alkaline slurry 23 is
introduced to the flue gas 20 via an inlet 26 (e.g., one or more
nozzles) in scrubber tower 22. As described above, aqueous alkaline
slurry 23 removes sulfur oxides from flue gas 20. Removal of
mercury salts is incident to this sulfur oxide removal process. The
cleansed flue gas 20 is released from scrubber tower 22 at outlet
32, where the flue gas 20 may flow to a stack or other emissions
control apparatus.
Aqueous alkaline slurry 23 is transported to scrubber tower 22 from
collecting tank 28 via one or more pumps 30. The amount of aqueous
alkaline slurry 23 transported to scrubber tower 22 varies
depending on several factors, including, but not limited to: the
amount of flue gas 20 present in the scrubber tower, the amount of
contaminants in the flue gas 20, and the design of the system 10.
After aqueous alkaline slurry 23 contacts flue gas 20 and removes
contaminants therefrom, the aqueous alkaline slurry 23 is collected
in collecting tank 28 for recirculation to inlet 26 by pump 30.
To control the mercury emission from the scrubber tower 22, a
measurement device 34 (e.g., a probe) measures the redox potential
of the aqueous alkaline slurry 23 in the collecting tank 28.
Measurement device 34 can be any device capable of measuring the
redox potential of aqueous alkaline slurry 23 present in collecting
tank 28. Examples of measurement devices include dissolved oxygen
analyzers, and probes. Measurement device 34 may measure the redox
potential of aqueous alkaline slurry 23 in collecting tank 28
either continuously or at predetermined intervals. For example, the
predetermined intervals may be determined automatically by a
control device 36, which is in communication with the measurement
device 34, or manually by a user.
After measuring the redox potential of aqueous alkaline slurry 23,
measurement device 34 provides a signal 38 indicative of the
measured redox potential to control device 36. Control device 36
may include, for example, a computer, a microprocessor, an
application specific integrated circuit, circuitry, or any other
device that can transmit and receive electrical signals from
various sources, at least temporarily store data indicated by such
signals, and perform mathematical and/or logical operations on the
data indicated by such signals. Control device 36 may include or be
connected to a monitor, a keyboard, or other user interface, and
includes an associated memory device 37.
Control device 36 compares the measured redox potential to one or
more predetermined redox potential values, which may be stored in
memory device 37. It is contemplated that the one or more
predetermined values may comprise a single value or a range of
values. The predetermined value(s) may be a user-input parameter.
For example, the predetermined redox potential value may be between
about 100 milli-volts (mv) and about 600 mv. By "predetermined" it
is simply meant that the value is determined before the comparison
is made.
Alternatively, the one or more predetermined redox potential values
may be determined by the control device 36 in response to output
signal 48 from a mercury measurement device 46, which measures the
amount of gaseous elemental mercury in flue gas 20 exiting from
scrubber tower 22. For example, if the output signal 48 indicates
that the emission of mercury is sufficiently low (e.g., below a
threshold mercury emission value stored in memory device 37), the
control device 36 can lower the predetermined redox potential
value, which results in a lower redox potential of aqueous alkaline
slurry 23 and, thus, a higher emission of mercury from scrubber
tower 22. If, on the other hand, the output signal 48 indicates
that the emission of mercury is too high (e.g., above the threshold
mercury emission value), the control device 36 can increase the
predetermined redox potential value, which results in a higher
redox potential and a lower emission of mercury.
Mercury measurement device 46 is any device that is suitable to
measure elemental mercury emitted from scrubber tower 22. Examples
include, but are not limited to: Continuous Emission Monitors
(CEMs), such as cold-vapor atomic absorption spectrometry (CVAAS);
cold-vapor atomic fluorescence spectrometry (CVAFS); in-situ
ultraviolet differential optical absorption spectroscopy (UVDOAS);
and atomic emission spectrometry (AES).
In response to the comparison of the measured redox potential to
the one or more predetermined redox potential values, the control
device 36 provides a control signal 42 to a means 40 for affecting
the reductive capacity of the aqueous alkaline slurry 23. In one
embodiment, the means 40 includes a forced oxidation system 41,
which adjusts an amount of oxidation air, such as an oxygen
containing gas 44, that is introduced into the aqueous alkaline
slurry 23 in the collecting tank 28 in response to the control
signal 42. Oxygen containing gas 44 can be any gas that contains
any amount of oxygen, for example air can be used as the oxygen
containing gas. Adjusting the amount of oxygen containing gas 44
introduced to collecting tank 28 adjusts the redox potential of
aqueous alkaline slurry 23 present in collecting tank 28.
For example, if the comparison of the measured and predetermined
redox potential values reveals that the measured redox potential
value is greater than the predetermined redox potential value,
control device 36 may provide a control signal 42 to the forced
oxidation system 41 to cause the forced oxidation system 41 to
decrease the amount of oxygen containing gas 44 being introduced to
collecting tank 28. Conversely if the comparison reveals that the
actual redox potential value is less than the predetermined redox
potential value, the controller may provide a control signal 42 to
the forced oxidation system 41 to cause the forced oxidation system
41 to increase the amount of oxygen containing gas 44 being
introduced to collecting tank 28. In this manner, it is possible to
limit the emission of mercury at the flue gas outlet 32, while
minimizing the consumption of oxygen containing gas. It is
contemplated that the control device 36 may employ known control
algorithms (e.g., proportional, integral, and/or derivative control
algorithms) to adjust the control signal 42 in response to the
comparison of the measured and predetermined redox potential
values.
Forced oxidation system 41 may employ a blower 43 of any suitable
type, which can introduce oxygen containing gas 44 into aqueous
alkaline slurry 23 present in collecting tank 28. In the example
shown, forced oxidation system 41 includes an inlet vane 45 which
operates to regulate the amount of oxygen containing gas 44
entering the blower 43 in response to the control signal 42 from
the controller 36. While the inlet vane 45 is a suitable device for
regulating the amount of gas 44 delivered to the tank 28, other
types of devices and methods could be employed, such as a valve
downstream of the blower 43, or by controlling the speed of the
blower 43. Alternatively, spargers, air lance agitators and
aspirators may be employed instead of a blower 43. Additionally,
forced oxidation system 41 may be connected to an agitator (not
shown) in collecting tank 28, which assists in distributing oxygen
containing gas 44 throughout aqueous alkaline slurry 23.
Although the subject matter has been described and illustrated with
respect to exemplary embodiments thereof, it should be understood
by those skilled in the art that the foregoing and various other
changes, omissions and additions may be made therein and thereto,
without parting from the spirit and scope of the disclosed method
and system. Accordingly, other embodiments are within the scope of
the following claims.
* * * * *